Among the variety of natural resources, the forest resource is an extremely significant resource of the Earth; in addition to producing diverse animal and plant ecology, the forest has also provided all kinds of resources required by humans and is closely related to our daily life. Therefore, the enhancement of the utilization rate, recovering rate, and recycling rate of wood has been a crucial topic requiring breakthroughs to be made for countries over the world, so as to reduce the unnecessary wasting of natural resources and effectively reduce the manpower and material resources required for processing scrap. In view of this, researches in wood-plastic composite (WPC) prepared using wood scraps or particles have received more and more attention in the world owing to the dimensional stability and insect resistance of WPC being superior to genuine wood in addition to the effective utilization of wood fiber scraps to fabricate WPC; therefore, WPC may be used for up to 25-30 years without particular maintenance. At present, the application of WPC is mainly in materials for transportation and non-structural and semi-structural architectures; for the example of United States, there are more than thirty WPC outdoor decking manufacturers and the market share of WPC outdoor decking has reached about 25%. Such indicates wider and popular utilizations and applications of WPC in the future.
Generally speaking, composite materials combine two or more raw materials with different characteristics by different processes to utilize the characteristics and advantages of each of the raw materials, and produce new materials having superior characteristics and satisfying requirements by synergistic effects. Among various composite materials, fiber-reinforced composite materials have superior characteristics and wider applications. The composite material is known as a wood-based composite when one of the raw materials is wood, and the fabrication of which mainly uses wood scraps or particles of different sizes to mix with adhesives and bond into boards under high temperature and high pressure. Conventional wood-based composites commonly use formaldehyde-based adhesives and issues of free formaldehyde often occur during usage; such wood-based composites also often expand and disperse when soaked into water. To solve such issues, wood-plastic composites (WPC) have been produced by mixing wood with plastic raw materials to reduce the release of free formaldehyde and improve the dimensional stability issues. In addition, since WPC combines the characteristics of wood and plastic, the shortcomings of each of the two raw materials may be overcome; for instance, plastic has the disadvantages of high specific weight, poor antistatic properties, and poor elasticity, yet the addition of wood reduces the specific weight and enhances antistatic properties and elasticity; on the other hand, for the wood raw material, the addition of plastic improves the dimensional stability, insect resistance, decay resistance, and weather resistance of the wood. Furthermore, when the percentage of added wood fiber increases, the structural strength or modulus of elasticity (MOE) of the material also increases.
Conventional WPC are fabricated by mixing wood flour or pulp materials with typical thermoplastics such as polyethylene, polypropylene, polyvinyl chloride, or polystyrene and form wood-fiber-plastic composite materials (or wood-flour-plastic composite materials) under high temperature. At present, approximately 70% of WPC uses polyethylene as the plastic matrix while lesser parts use polypropylene and polyvinyl chloride as the plastic matrix. In addition, to enhance the strength and ductility of thermoplastic products and reduce production cost, fibers and fillings are often added to the plastic as intensifiers; most of the early plastic industries (about 93%) use synthetic fibers for the fibers, while recycled wood fibers have become more common in the recent years. The oil crisis in the 1970s has also encouraged the usage of biomass fibers in the industry.
In summary, WPC substitutes the inorganic fiber materials or fillings in conventional plastics with wood fibers, and when the wood fibers are mixed with thermoplastics to yield composite materials, not only the physical characteristics and mechanical properties of wood materials are effectively improved but also the processability is enhanced. Therefore, WPC has largely replaced conventional metal, plastic, and solid wood materials in recent years and is commonly applied in products of decking, fences, outer panels, frames, and roofs. However, several issues exist when wood fiber (or wood flour) is used as an intensifier or filling for plastics, and the main issue among which is that the difference between the surface polarities of plastics and wood fibers hinders the chemical bonding between the two; furthermore, wood fibers suffer from poor dispersion in plastics due to hydrogen bonds between the fibers.
To solve the compatibility issue of plastic and fibers, coupling agents or adhesion-promoting agents such as maleated polyethylene (MAPE), maleic anhydride grafted polypropylene (MAgPP), and oxidized LDPE may be used to overcome this issue; however, to solve the poor dispersion issue of fibers requires adding dispersing agents such as stearic acid, paraffin wax, or mineral oil to achieve improvements. In addition, grafting may be utilized to form copolymers of monomers (e.g., 1-phenylthene, 4-methyl-2-oxy-3-oxopent-4-ene, etc.) and wood fibers to solve the issues of fiber polarity and dispersion. Chemical modification is also used in recent years to improve the interface properties of WPC. Generally speaking, although chemical modification may reduce the surface polarity of WPC and enhance the compatibility of wood and plastic, it requires using many chemicals, where the most basic chemical modification uses modifiers with a single functional group; modifiers with double functional groups or multiple functional groups may also be used. Among various chemical modification methods, the most appreciated and practical one is the method of reacting acetic anhydride with hydroxyl groups of the wood, i.e., acetylation; however, when carrying out acetylation reactions by acetic anhydride, longer reaction times are required to obtain superior weight percentage gain (WPG) or degree of substitution when under environments without solvents and catalysts; therefore, solvents such as dimethylformamide (DMF), dimethyl sulfoxide (DMSO) or pyridine are commonly used when carrying out acetylation reactions by acetic anhydride to enhance the reaction rate with acetic anhydride so that superior acetylation results may be achieved in shorter reaction times. However, the addition of solvents inevitably increases cost and toxicity resulting from the solvent, while the reaction time still takes several hours to achieve a superior acetylation result.
The rising environmental consciousness in recent years has impeded the popular usage or application of the aforementioned WPC or other plastic composites in our daily needs due to the various structural disadvantages and toxicity issues described in above. Thusly, the development of an emulated wood with high elasticity and strength, non-toxicity, recyclability, good dyeability, weather resistance, etc. and further emulating the wood characteristics and wood grain of genuine wood and having plasticity and fiber strength superior to those of genuine wood has been a crucial topic to be solved for manufacturers over the world.